I'm not sure what it is that holds my interest in these early computers. Maybe it's because these were the first computers I could work with at home. Or maybe it's just the ultimate simplicity of these computers, in the age of ever growing technology. Maybe it's just because they are fun to play with. I can program them to handle simple jobs that don't require a full blown computer.
In the early days of computers, nobody had a computer that they could call their own. The computers they used were very expensive and usually belonged to some company. But in the early 70s, entry level computers were introduced that were cheap enough for someone to have one at home. Many of the top chip makers, like Motorola, started releasing kits that allowed the user to understand what a computer is made from and how to program it.
Motorola MEK6800-D1
My introduction was a Motorola MEK6800-D1. This was a MC6800 based computer with a MC6810 RAM (128 Bytes of memory), two MC6820 PIA (Peripheral Interface Adapter) chips, and one MC8650ACIA (Asynchronous Communication Interface Adapter) chip. There was also a MC6830 ROM (Read Only Memory) that contained a monitor program. This program used one of the PIAs as a serial communication device that could talk to a dumb terminal. This gave the user immediate access to the monitor program. The monitor program contained single character commands that allowed the user to read/write memory and set up for a data transfer to a storage device. The storage device in this case was a cassette tape recorder.
A plus, for me, was that my employer had purchased all of the printed material that goes along with the MEK6800-D1. This was books like the Motorola User Group Library and various chip manuals. All I had to do was absorb everything that was printed in these books. And I certainly did my best. But, while these were great kits for uses to learn the basics of computers, the technology of the day moved very quickly. So all of a sudden, these simple kits were no longer useful. Once they were out of use, employers didn't want them around and gave them to whoever wanted them. That would be me.
EPA Micro-68
Over time, I had also collected a more sophisticated entry level computer than the MEK6800-D1. I obtained a EPA Micro-68. The EPA Micro-68 was also a MC6800 based computer, but with extra options. Rather than being just a small circuit board, that required power supplies and a communication terminal, The EPA Micro-68 came with a nice wooden case, a plugable backplane, and a built in power supply. The main computer board also had a hex keyboard and some LED character display ICs. The monitor program was then modified to interface with keyboard and displays so that the whole computer was completely stand-alone. As an option, 8K Byte memory boards and cassette interfaces were also available. The memory boards plugged directly into the backplane and the cassette interface installed near the keyboard. Everything you needed was right there. Of course, a external dumb terminal interface was also included.
The image shows a top view of the EPA Micro-68 Main Computer Board without it's cover and not plugged into the backplane. Clicking on the image will bring up a higher resolution image. You might note that there are six empty sockets for RAMs, next to the CPU. Stand alone, these sockets could be populated with MC6810 128 Byte Rams. That would give the user 768 Bytes of RAM. However, I have a 8K Byte memory board that also plugs into the backplane. This eliminates the need for the on-board RAMs. The empty board space above the keyboard is where the Cassette Interface installs. I had it out when the picture was taken. The perf board, with the 4 ICs, is a address fix for the MC6820 PIAs.
Also note, on the left side of the main computer board, above right, are components for a +5 Volt Power Supply. However, it is missing a transformer and +5 Volt regulator. If you wanted to run the main computer board stand alone, you could add a transformer and +5 Volt regulator.
The image on the left is the chassis. Clicking on the image will bring up a higher resolution image. On the back of the chassis is the back plane with one connector for the computer and four connectors for system expansion. The computer, from the other image, sits on top of the chassis and plugs into the top back plane connector. Note that the bottom connector on the back plane really can't be used.
On the bottom aluminum plate are the power supplies. There are two transformers. One (bottom center in the chassis) is used for the +5 Volt power supply and the other (top left in the chassis) handles the +12 Volt and -12 Volt supplies. Not easily visible are the +5 Volt regulator and four pass transistors. Two pass transistors are for the +5 Volt and the other two are for the +12 Volt and -12 Volt power supplies. That's because they are mounted on the bottom aluminum plate. Also on the bottom aluminum plate is the On/Off Switch and Fuse.
Software
Initially, working with these computers was a bit clumsy. In order to get the program into the computer, you had to first write the program in Pseudo Code (a.k.a. Assembly Language). Then the Pseudo Code needed to be converted to 1, 2 or 3, "two digit" hexidecimal values and manually entered into the computer. These hexidecimal values were the actual instructions and data for the computer.
If you didn't want to assemble and enter your program manually, assembly programs were available. For example, Motorols developed a Resident Assembler and Editor. This assembler/editor could be loaded directly into your MC6800 computer. Then you can write, edit, assemble, and install your programs. There were utilites in the assembler/editor that would allow you to save/restore your program to/from cassette tape. Of course, you needed at least and 8K Byte memory expansion to use the assembler/editor.
Cross-Assemblers were also available. A Cross-Assemblers was designed to run on a computer with a different operating system. It would allow the user to read, write, edit, assemble on that system. An output file could then be loaded into the MC6800 computer. Often, this required the user to output the assembled program to cassette tape. Then it could be loaded into the MC6800 computer.
Personally, I found most commercial Cross-Assemblers difficult/clumsy to use. So I wrote my own giving it features that I thought the assembler needed. I wanted the assembly of a program to clean up the source code, flag errors, resolve forward references, and be self documenting.